This is in preparation for DND.
It moves a lot of code from gdkclipboard-x11.c to
gdkselectionoutputstream-x11.c to untangle it from GdkX11Clipboard
usage.
This is the replacement for selection usage.
Backend implementations for X11 (missing support for backwards compat
formats like COMPOUND_TEXT) and Wayland are included.
GTK code should be adapted to use gdk_drop_read_*() functions instead
of gtk_drag_get_data().
When the reply to a TARGETS request comes in, the clipboard may already
be reclaimed by the local app. Deal with that case (in an ugly way,
strictly speaking we should use a cancellable here).
This happens for example at startup when the initial TARGETS requests
have not been answered until after the main widow popped up. And if such
a window immediately claims the primary clipboard (like when the initial
focus is inside an entry), this race will happen.
This object tracks the SelectionNotifyEvent that has to be sent in
response to a SelectionRequest.
Currently it just looks like code reshuffling, but it's a prerequisite
for handling MULTIPLE, which requires to only send the notify after
every stream has writtten at least once.
But anyway, code is cleaner now, so it's a win!
(1) Try all passed in formats in order if one of them fails.
(2) Don't blindly accept all formats, make sure they are mime types
(3) Add a bunch of special non-mime types that plug converters to
get to mime types
This allows us not just to pass any mime type to the read function, but
it also makes it possible to pass multiple mime types and the clipboard
can then try them in order until it finds a supported one.
This is so far not implemented though.
(1) Turn X11 clipboard event handling into a regular filter function
(2) Maintain a timestamp in the clipboard, so we can pass it when
querying selections.
